X-ray photoemission system for 3-D laminography

ABSTRACT

A system is disclosed for the examination and inspection of integrated devices such as integrated circuits using 3-D laminography. X-rays are transmitted through the integrated device, and are incident on a photoemissive structure that absorbs x-rays and emits electrons. The electrons emitted by the photoemissive structure are shaped by an electron optical system to form a magnified image of the emitted electrons on a detector. This magnified image is then recorded and processed. In some embodiments, the incidence angle of the x-rays is varied to gather multiple images that allow internal three-dimensional structures of the integrated device to be determined using computed laminography. In some embodiments, the recorded images are compared with reference data to enable inspection for manufacturing quality control.

RELATED INVENTIONS

The present Application is a Continuation of U.S. patent applicationSer. No. 16/690,103, filed Nov. 20, 2019, entitled X-RAY PHOTOEMISSIONAPPARATUS FOR INSPECTION OF INTEGRATED DEVICES, which is scheduled toissue as U.S. Pat. No. 11,307,152 on Apr. 19, 2022, and is in turnContinuation of U.S. patent application Ser. No. 15/076,556, filed Mar.21, 2016 and entitled INTEGRATED DEVICES WITH PHOTOEMISSIVE STRUCTURES,which in turn is a Divisional of U.S. patent application Ser. No.13/507,895, filed Aug. 3, 2012 and issued on Mar. 22, 2016 as U.S. Pat.No. 9,291,578 entitled X-RAY PHOTOEMISSION MICROSCOPE FOR INTEGRATEDDEVICES, all of which are herein incorporated by reference in theirentirety.

FIELD OF THE INVENTION

This invention relates to the examination of integrated devices, such asintegrated circuits, by transmitting x-rays through the device andmagnifying the resulting images; and in particular, to the use of ahybrid system which converts the transmitted x-rays to electron-beams,which are then magnified using electron optics for the resolution ofphysical structures much smaller than 100 nm in size. The particularembodiments disclosed here allow for the observation of the device atmultiple angles to determine the two-dimensional and three-dimensionalstructures within the device without physically damaging the device,and, when paired with a reference image or a reference database, canalso be used as an inspection system for devices of unknown quality.

BACKGROUND OF THE INVENTION

The initial discovery of x-rays by Röntgen in 1895 [W. C. Röntgen, “EineNeue Art von Strahlen” (Würzburg: Verlag und Druck der Stahel'schen K.Hofund Universitäts-Buch- und Kunsthandlung, Würzburg, Germany, 1896);“On a New Kind of Rays,” Nature, Vol. 53, pp. 274-276 (Jan. 23 1896)]was in the form of shadowgraphs, in which the contrast of x-raytransmission for biological samples (e.g. bones vs. tissue) allowedinternal structures to be revealed without damaging the samplesthemselves. However, because of their short wavelength (10 to 0.01 nm,corresponding to energies in the range of 100-100,000 eV), and theabsence of materials for which the refractive index for x-rays differssignificantly from 1, there are no easy equivalents to refractive orreflective optical elements so commonly used in optical system design.So, even now, the most common use of x-rays is still as a simpleshadowgraph, observing the structure of bones and teeth in the officesof doctors and dentists.

Early x-ray “microscopy,” developed more than 50 years after the initialdiscovery of x-rays, simply consisted of elaborate shadowgraphapparatus, in which the diverging x-rays cast a shadow larger than theobject [S. P. Newberry and S. E. Summers, U.S. Pat. No. 2,814,729]. Withthe advent of computer data collection, it became possible to gathermore information from the specimen, changing the relative positions andillumination angles of the x-ray source and specimen in a systematicway. Using multiple transmission measurements taken at multiple anglesaround the specimen, images can be synthesized by computer thatrepresent a 2-dimensional or 3-dimensional model of the specimen [G. N.Hounsfield, U.S. Pat. No. 3,778,614]. The “slices” of interior bodies sorevealed are amazing to look at, revealing a great deal about theinternal structures without invasive surgery. However, as far as thephysics of the x-ray interaction with the specimen, these tomographicreconstructions represent nothing more than an elaborate map of x-rayabsorption—a sophisticated shadowgraph.

Over time, other imaging tools for x-ray optical systems were invented.Apparatus using grazing incidence reflection off of surfaces providedcone reflectors [C. G. Wang, U.S. Pat. No. 4,317,036] and capillarycollimators [F. Kumasaka et al., U.S. Pat. No. 5,276,724] to allow adiverging x-ray beam to be manipulated into a collimated beam or toconcentrate x-rays onto a specimen.

With the development of high-resolution patterning with electron-beamlithography in the 1970's, Fresnel zone plates, which use diffractiveproperties to effectively focus an electromagnetic wave, could now bemanufactured at the small dimensions suitable for use with short x-raywavelengths. [J. Kirz, “Phase zone plates for x rays and the extremeuv”, Journal of the Optical Society of America, Vol. 64(3), pp. 301-309(March 1974)]. Zone plates can be used both to shape and focus theilluminating optics and also to collect and focus the transmitted x-raysonto a detector [G. Schmahl and D. Rudolph, “X-Ray Microscopy” pp.192-202, (Springer Verlag, Berlin, 1984); and U.S. Pat. No. 4,870,674].Variations using phase-contrast rings [G. Schmal [sic] and D. Rudolph,U.S. Pat. No. 5,550,887] have been developed, and are now commonly usedin contemporary x-ray microscopes.

Unfortunately, what a zone plate microscope design may have inresolution may not be matched in imaging speed. The diffractiveproperties of the zone plate are tuned to a specific wavelength, meaningthat most of the energy in a broad-band x-ray source is discarded.Synchrotron sources may increase brightness for a particular wavelength,but are not suitable for portable systems and, at the selectedwavelength, the best diffraction efficiency that can be achieved isstill under 35%.

Because of this, the microscopy of specimens requiring high speed andhigh resolution use electron microscopy instead, either as scanningelectron microscopes (SEMs) or transmission electron microscopes (TEMs).Being charged particles, electrons can be easily controlled and focusedusing electric and magnetic fields, and the science and technology ofelectron optics is a well-developed and established field. [L. Reimer,“Electron Optics”, Section 2 of Ch. 2 of “Scanning Electron Microscopy:Physics of Image Formation and Microanalysis, 2^(nd) Edition”, (SpringerVerlag, Heidelberg, 1998)].

Electron beams require that the sample and the beam path must all be ina vacuum. Since any sample would lose all its water in the desiccatingenvironment of a vacuum chamber, this does not represent a way ofobserving most biological samples in their “natural” condition. Also,depending on their energy, electrons tend to be absorbed with the firstfew nanometers of a sample, making them extremely useful for theobservation of surfaces, but not so useful for the observation ofinternal structures. Samples must be thinned to be less than 100 nmthick, and often only a few tens of nm thick, before they can be used ina TEM.

In an attempt to combine the penetrating power of x-rays with thecontrol and resolution possible with electron-beams, a hybrid of x-raymicroscopy and photoemissive electron microscopy, or PEEM, has beendeveloped [O. H. Griffith and W. Engel, “Historical perspective andcurrent trends in emission microscopy, mirror electron microscopy andlow-energy electron microscopy,” Ultramicroscopy, Vol. 36, p. 1 (1991)].Although PEEM is usually a technique in which a surface is excited fromthe front and photoelectrons also emitted from the same front surface, aphotocathode mounted on a sufficiently thin membrane can allowexcitation from the back side through a membrane [H. Hirose, U.S. Pat.No. 5,045,696].

FIG. 1 illustrates a prior art hybrid x-ray/PEEM system as disclosed byF. Cerrina and T. B. Lucatorto on Drawing Sheet 2 of U.S. Pat. No.6,002,740. In this system, described as being a system to inspect masksfor x-ray lithography, the mask 22 is placed between a source of x-rays30 and converter 18 comprising a photo-emitting cathode 16 mounted on amembrane 19. When the converter 18 is illuminated through the membrane19 by x-rays, it emits electrons 32 whose intensity is “directlyproportional to the local intensity of the x-rays impinging thereon.”

The electrons 32 emitted from the converter 18 are then highly magnifiedby a set of electron optics in the electron microscope 17. The electronmicroscope 17 forms an image of the mask pattern that may be fed to thecomputer system 20 for analysis and display.

The Cerrina disclosure describes a hybrid x-ray/PEEM inspection systemfor x-ray lithography masks, in which the system emulates an x-raylithography system. [H. Smith and M. Schattenberg, “X-ray lithographyfrom 500 to 30 nm,” IBM Journal of Research and Development, Vol. 37(3),p. 319 (1993)]. The configuration described requires placing thephotoemitting cathode relative to the mask in the same position that aphotoresist-coated wafer would be placed in an x-ray lithography system,allowing the image to mimic what the mask would print. In such alithography system, both the mask and the wafer are placed in a vacuumin close proximity for proximity printing, with a distance of less than25 microns separating them to minimize distortions, [A. D. Dubner etal., “Diffraction effects in x-ray proximity printing,” Journal ofVacuum Science and Technology B, Vol. 10(5), pp. 2234-2242 (1992)] butnot in direct contact to avoid damaging the mask or wafer.

Such hybrid systems were proposed but never applied to x-raylithographic mask inspection because x-ray lithography did not achieveany widespread commercial adoption. Such systems have been built anddemonstrated for various biological and mineral samples. [R. N. Watts etal., “A transmission x-ray microscope based on secondary-electronimaging,” Review of Scientific Instruments, Vol. 68, p 3464 (1997); G.De Stasio et al., “Soft-x-ray transmission photoelectronspectromicroscopy with the MEPHISTO system,” Review of ScientificInstruments, Vol. 69, p. 3106 (1998), and “MEPHISTO spectromicroscopereaches 20 nm lateral resolution,” Review of Scientific Instruments,Vol. 70 , p. 1740 (1999); Y. Hwu et al., “Using photoelectron microscopywith hard x-rays,” Surface Science, Vol. 480, pp. 188-195 (2001)].However, many biological structures are well observed by variations ofconventional optical and x-ray tomographic tools, making the complexityof these hybrid systems unnecessary for many biological applications.

But, for one particular class of specimens, variations on this hybridtechnique may be perfectly suited, and are the subject of the inventiondisclosed here.

One problem that has recently emerged is the need to examine productscontaining integrated devices, such as integrated circuits (ICs), toverify that the devices have been manufactured as specified. This isespecially important when the security and integrity of the devices maybe an issue, in which is it necessary to insure that additionalcircuitry (e.g. RF antennas to relay signals from unauthorized sources)have not been inserted during the manufacturing process. When allcircuit structures are encased within a single package, verification ofthe actual contents of the circuit is difficult.

Current examination techniques for these circuit packages requiredestructive testing, taking the circuit package and removing materiallayer by layer, photographing and analyzing the circuit patterns of eachlayer as they are exposed with either an optical microscope, or with anelectron microscope for smaller structures. This can be very tedious andtime consuming. With the components of the most modern ICs quicklyapproaching 20 nm in size, and potentially becoming as small as 5 nm infuture generations, there is a real need for an imaging technique whichhas the resolution to identify these small features and also the speedto observe multiple layers of devices and interconnects over a 1 cm by 1cm area in a manageable amount of time.

An approach using the transmissive power of x-rays to examine theinternal contents of a circuit will not require the destruction of thecircuit itself, and has the potential to provide both the resolutionneeded and the speed required.

Systems using an x-ray microscope for the inspection of integratedcircuits have been disclosed by the Xradia Corporation [W. Yun and Y.Wang, U.S. Pat. No. 7,119,953; Y. Wang et al., U.S. Pat. No. 7,394,890;M. Bajura et al., U.S. Pat. No. 8,139,846; www.xradia.com]. FIG. 2illustrates a prior art x-ray microscope system as disclosed on DrawingSheet 2 of U.S. Pat. No. 7,119,953. In such a system, x-rays from asource 1110 are collected by a condenser 1120, which relays x-rays fromthe source 1110 to the test object 1010. This condenser 1120 isdescribed in some embodiments as a capillary condenser with a suitablyconfigured reflecting surface, while in others as a zone plate. Theconverging beam from the condenser 1120 irradiates the test object 1010,and the radiation emerging from the test object 1010 is scattered anddiffracted out of the path of the direct radiation beam. An objective1118 is therefore used to form an image of the object, collecting thescattered x-rays. This objective 1118 is described as being possibly azone plate lens, a Wolter optic, or a Fresnel optic. In someembodiments, an additional phase plate 1116, often in the form of a ringaround the center axis of the system, is included to enhance contrast.Both the phase plate 1116 and the objective 1118 are described as beingattached to a “high-transmissive substrate” 1140 to form a compositeoptic 1138. The image of the test object 1010 is formed on a detector1125, which is described as possibly comprising in some embodiments acharged coupled device (CCD), and in some embodiments comprising ascintillator, and in others being a film-based detector.

X-ray systems with Fresnel zone plate (FZP) optics such as this priorart Xradia system can be effective for the non-destructive examinationof integrated circuits, but the limitations of the zone plate optics [J.Kirz and D. Attwood, “Zone Plates”, Sec. 4.4 of the “X-ray Data Booklet”(xdb.lbl.gov/Section4/Sec_4-4.html) ] reduce the wavelength range overwhich x-rays can be effectively collected, and increase the time tocollect data for a complete IC. The system is therefore very slow andinefficient for collecting large volumes of data on multiple layers ofan IC.

There is therefore a need for a system that can combine the penetratingpower of x-rays with the easy control possible in electron imaging, andin particular for the application to the microscopy of sub-100 nmstructures in integrated circuits to allow rapid, non-destructivetesting and inspection of those integrated circuits.

BRIEF SUMMARY OF THE INVENTION

The invention disclosed with this application is an apparatus for theexamination and inspection of an integrated device such as an integratedcircuit. In this invention, a photoemissive structure placed in a vacuumchamber converts incident x-rays, which have been transmitted throughthe integrated device, into the emission of electrons, and the electronsemitted by the converter are shaped by an electron optical system toform a magnified image of the emitted electrons on a detector.

In preferred embodiments of the invention, the x-ray intensity patternincident on the photoemissive structure will have a profile representingthe attenuation of x-rays in the integrated device under examination,and the materials of the photoemissive structure will be selected sothat the number of electrons emitted are proportional to the intensityof the incident x-rays.

The magnified image produced by the detector can then be recorded andprocessed. In some embodiments, the image is compared with acorresponding image of a device known to be correct. In anotherembodiment, the image is compared to a database representation of thestructures in the circuit.

In yet another embodiment, the integrated device under examination ismounted on a stage and the incident x-rays are moved through a series ofangles and positions relative to the integrated device, and a set ofcorresponding transmission images recorded. These images can then beassembled using computed laminography algorithms with a digital computerto create a 2-D or 3-D representation of the specimen. This synthesizedrepresentation can then be compared to a reference image or database,allowing the embodiment to be used as an inspection system.

In some embodiments of the invention, the integrated device underexamination is mounted outside the vacuum chamber containing thephotoemissive structure and the electron optics.

In other embodiments of the invention, the photoemissive structure iscoated directly onto the window of the vacuum system that contains theelectron optics, reducing the distance between the specimen and thephotoemissive structure.

In other embodiments of the invention, the specimen to be examined isdirectly coated with the photoemissive layer, and mounted within thevacuum system containing the electron optics, and is illuminated byx-rays through a suitably transparent window in the wall of the vacuumchamber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art hybrid x-ray/PEEM system as disclosed inU.S. Pat. No. 6,002,740.

FIG. 2 illustrates a prior art x-ray microscope system from the XradiaCorporation as disclosed in U.S. Pat. No. 7,119,953.

FIG. 3 illustrates a cross-section view of a microscope system accordingto one embodiment of the present invention, in which the integrateddevice is mounted outside the vacuum chamber and the photoemissivestructure is mounted within the vacuum chamber.

FIG. 4 illustrates a detailed cross-section view of the integrateddevice and photoemissive structure for the embodiment illustrated inFIG. 3 .

FIG. 5 illustrates a typical copper x-ray absorption spectrum.

FIG. 6 illustrates a detailed cross-section view of the electron imageconverter and imaging system for the embodiment illustrated in FIG. 3 .

FIG. 7 illustrates a schematic of the control systems to be used for theembodiment illustrated in FIG. 3 .

FIG. 8 illustrates a cross-section view of a microscope system accordingto a second embodiment of the invention, in which the integrated deviceis outside the vacuum chamber and the photoemissive structure is coatedonto the window of the vacuum chamber.

FIG. 9 illustrates a detailed cross-section view of the integrateddevice and photoemissive structure for the embodiment illustrated inFIG. 8 .

FIG. 10 illustrates a cross-section view of a microscope systemaccording to a third embodiment of the invention, in which theintegrated device is inside the vacuum chamber and the photoemissivestructure is coated onto the integrated device.

FIG. 11 illustrates a detailed cross-section view of the integrateddevice and photoemissive structure for the embodiment illustrated inFIG. 10 .

FIG. 12 illustrates a detailed cross-section view of the integrateddevice and photoemissive structure for a variation of the embodimentillustrated in FIG. 10 in which the angle of incidence is variable.

FIG. 13 illustrates a detailed cross-section view of the integrateddevice and photoemissive structure for a variation of the embodimentillustrated in FIG. 10 .

FIG. 14 illustrates a detailed cross-section view of the integrateddevice and photoemissive structure for a fourth embodiment of theinvention.

Note: The elements in the drawings illustrate the elements of theinvention and their general relationships, but should not be interpretedas scale drawings. For example, in FIG. 3 , the entrance window 125 inthe vacuum chamber 120 may be only a few mm in diameter or smaller,while the entire vacuum chamber 120 may be as long as a meter, but theseelements have not been shown at these relative dimensions here.Likewise, in FIG. 4 , in an integrated device 160, the silicon substrate162 may be 500 microns thick, whereas the layer comprising integratedstructures 164 may be only 10-20 microns thick. The illustrations doshow the general relationships sufficiently so that one skilled in theart would be able to reproduce the invention accordingly.

Note: The cross-section views have been selected to represent elementsin a plane in which the x-rays and emitted electrons are traveling. Someof the elements also presented in the cross-section views, and inparticular the stage controls 132 and 332 inside the vacuum chamber 120as well as the external stage controls 232 would typically be, at leastin part, below or above the plane of the illustrated cross-section,especially for the region through which the x-rays or emitted electronsare traveling, so that these mechanical elements will not block thex-rays. However, the illustrations and descriptions in the specificationpresent the general relationships sufficiently so that one skilled inthe art would be able to reproduce the invention accordingly.

DETAILED DESCRIPTIONS OF EMBODIMENTS OF THE INVENTION First Embodimentof the Invention

One embodiment of the apparatus according to the invention for theexamination of an integrated device 160 is illustrated in FIG. 3 throughFIG. 7 . FIG. 3 illustrates a cross section view of the overall system.FIG. 4 illustrates a cross-section in detail of the integrated device160 and photoemissive structure 170. FIG. 5 illustrates the x-ray energyabsorption for copper, used in many integrated devices. FIG. 6illustrates a cross section in detail of the image converter 180 and theimaging system 190. FIG. 7 illustrates a schematic of the variouselements connected to the system controller 110.

Turning first to FIG. 3 , the apparatus comprises a source of x-rays 100and also comprises beam-shaping optics 104 which provide a beam ofx-rays 111, directed onto the integrated device 160, with apredetermined energy (i.e. wavelength) spectrum and a predeterminedangular distribution. The energy spectrum can be a broad emissionspectrum, or filtered to have a specific set of wavelengths, or cancomprise some other combination of wavelengths. Likewise, the angulardistribution can be a diverging beam, a converging beam, or a collimatedbeam.

The source of x-rays 100 can be any x-ray source, including asynchrotron, a fixed target x-ray tube, a rotating anode source, a laserplasma source, or other sources that will be well known to those skilledin the art. The source of x-rays 100 can be operated to continuouslyemit x-rays, or be operated in a pulsed mode. The beam shaping optics104 can comprise any of a number of x-ray beam shaping tools, includingcapillary collimators, grazing incidence reflecting cones, and zoneplates. However, a beam of x-rays for this system would generally have anumerical aperture of approximately nine milliradians (9 mrad).

The system further comprises a vacuum chamber 120 and a means ofestablishing a vacuum 121 within the vacuum chamber 120, such as avacuum pump. The means of establishing a vacuum 121 may use a valve 122for vacuum control, which in some embodiments of the invention can allowthe vacuum chamber to be detached from the means of creating a vacuum121 once the vacuum is established. Additionally, the wall of the vacuumchamber 120 may comprise junctions 123 to preserve vacuum at anymechanical or electrical access points in the system, and vacuum seals124 at other access points such as windows.

In this embodiment of the invention, on the side of the system exposedto the beam of x-rays 111, the integrated device 160 is mounted outsidethe vacuum chamber 120, using an external stage 230. The external stage230 may be attached to the vacuum chamber 120 or, in some embodiments,may be independently supported and not in physical contact with thevacuum chamber. In some embodiments of the invention, this externalstage 230 will be fixed in place, and in others the external stage 230may have external stage controls 232 to adjust position and orientation.Adjustments enabled for the external stage 230 through the controls 232may include motion in the x-y plane perpendicular to the axis ofpropagation of the x-rays; it may also include rotation about the x- ory-axis, and it may also include translation in the z-axis along the axisof propagation of the x-rays, and may also include rotation around thez-axis. It may also include rotation and/or translation about any axisor axes.

Adjustment of the external stage controls 232 can be governed by anexternal stage controller 234, which is generally a system ofelectronics also outside the vacuum chamber 120. In some embodiments,this external stage controller 234 can in turn be controlled by theoverall system controller 110, which can designate an organized scan ofpositions and orientation angles of the external stage 230 to facilitatethe examination of the entire integrated device 160 at multiple exposureangles. These controls may make it possible for a relatively small beamof x-rays to be used to examine the entire area of an integrated device160 much larger than the diameter of the x-ray beam 111.

After passing through the integrated device 160, the intensity of thebeam of x-rays 111 will be modified by the absorption or scattering ofx-rays within the integrated device 160. The modified x-ray beam 211enters the vacuum chamber 120 through an entrance window 125, made froma material selected to be relatively transparent to x-rays, such asberyllium or diamond. It is desired that this be a uniform material, sothat the intensity profile of the internal modified beam of x-rays 311(i.e. inside the vacuum chamber 120) is proportional and nearlyidentical to that of the external modified beam of x-rays 211.

Once inside the vacuum chamber 120, the internal modified beam of x-rays311 will encounter a photoemissive structure 170. Some x-rays will beabsorbed by the photoemissive structure 170, while the remainingunabsorbed beam of x-rays 511 exits the photoemissive structure 170 andproceeds on into the vacuum chamber 120. In some embodiments of theinvention, the unabsorbed beam of x-rays 511 will eventually be absorbedby a beam dump 108 elsewhere in the vacuum system.

The absorption of x-rays in this structure stimulates the emission ofelectrons 179 from the photoemissive structure 170. However, since theelements of the photoemissive structure 170 are often thin films, insome embodiments of the invention a support structure 270 may also beused provide additional mechanical support for the photoemissivestructure 170. Care should be taken in the selection of materials forthe support structure 270 so that significant distortions to theinternal modified beam of x-rays 311 are not introduced.

The support structure 270 and photoemissive structure 170 can besupported and adjusted using a stage 130 within the vacuum chamber 120.This stage 130 may be attached to the vacuum chamber 120, as illustratedin FIG. 3 . In some embodiments of the invention, this stage 130 will befixed in place, and in others the stage 130 may have stage controls 132to adjust position and orientation. Adjustments enabled for the stage130 may include motion in the x-y plane perpendicular to the axis ofpropagation of the x-rays; it may also include rotation about the x- ory-axis, and it may also include translation in the z-axis along the axisof propagation of the x-rays, and may also include rotation around thez-axis.

Adjustment of the stage controls 132 can be governed by a stagecontroller 134, which is generally a system of electronics also outsidethe vacuum chamber 120. In some embodiments, this stage controller 134can in turn be controlled by the overall system controller 110, whichcan designate an organized scan of positions and angles of the stage 130to improve the signal from the emitted electrons 179.

FIG. 4 shows a detailed cross-section view of this embodiment of theintegrated device 160 and photoemissive structure 170 according to theinvention in more detail. A typical integrated device 160, such as anintegrated circuit, will comprise a substrate 162 of a material, such assilicon, on which a layer comprising integrated structures 164 has beenfabricated. The devices will often be in the form of a plurality ofstructures, often fabricated in several planar processing steps thatform devices and the structures interconnecting them. The illustrationof FIG. 4 shows a cross-section of a representation of a layercomprising integrated structures 164 which contains many structuresfabricated using 4 planar layers, which include dielectric material 165,shown as grey, and numerous metal interconnect structures 166, shown asblack. For simplicity, only a few structures have been illustrated inFIG. 4 , but for many contemporary integrated circuits, the metalinterconnect structures are often manufactured using copper and thenumber of metal interconnect structures number in the billions. Typicaloverall x- and y-dimensions for an integrated device 160 may be 1 cm×1cm, while the dimensions of the interconnect structures 166 are oftensmaller than 100 nm, and can be as small as 20 nm in a contemporaryintegrated circuit.

For some embodiments of the invention, especially when used to observecopper interconnect structures, the energy of the beam of x-rays 111 canbe selected so that a significant portion of the x-rays have energygreater than the energy of the copper K-band absorption edge. FIG. 5illustrates a graph showing a typical plot of the copper x-rayabsorption near edge structure (XANES) absorption spectrum. When thespectrum of x-ray energy for the beam of x-rays 111 is chosen to have asignificant fraction above the copper K-band absorption at 8.98 eV, thecopper structures will strongly absorb many of the x-rays, while theother dielectric structures will transmit the x-rays. This will resultin the varying degrees of x-ray transmission through the integrateddevice and therefore improved contrast. The external modified beam ofx-rays 211 will therefore have a strongly varying intensity profile,representing copper interconnect structures.

Returning to FIG. 4 , The photoemissive structure 170 may comprise alayer 172 of photoemissive material that generates a cascade ofelectrons 177 when irradiated with x-rays. This layer 172 can befabricated using a material such as gold, but other photoemissivematerials will be known to those skilled in the art of electron-materialinteractions. It is desired that the composition of the material of thislayer 172 be relatively uniform, so that the cascade of electrons 177,comprising primary electrons generated by the x-rays and also secondaryelectrons generated in turn by the primary electrons, has an electrondensity that is in proportion to the local flux of x-rays passingthrough the material.

The thickness of the layer 172 must also be selected with care, as alayer 172 that is too thin may not generate a strong cascade ofelectrons 177, while a layer 172 that is too thick may generate acascade of electrons 177 whose number is no longer in proportion to theflux of incident x-rays. In one embodiment of the invention, layer 172is fabricated using gold having a thickness of approximately 50 nm.

Once the cascade of electrons 177 is generated, some of these electronsexit the layer 172 of photoemissive material. However, in some cases,the material used to fabricate the layer 172 of photoemissive materialmay be selected to have the capability of generating a large number ofelectrons from a few absorbed x-rays, but which may also have a largework function. This may reduce the emission of some of the generatedelectrons, which can lead to a reduction in the overall signal strength.

Therefore, in some embodiments of the invention, it is desired that thephotoemissive structure 170 additionally comprise an emissive coating174. The material used to fabricate this emissive coating 174 can bechosen to have a low work function, so that it is more likely that thecascade of electrons 177 initiated by the x-ray exposure in the layer172 of photoemissive material will result in a large number of emittedelectrons 179. The material used for the emissive coating 174 can alsobe chosen so that the electrons of the cascade of electrons 177 that aretransferred into the emissive coating 174 can generate additionalsecondary electrons, forming an amplified electron cascade 178.Materials such as cesium iodide (CsI), which has a low work function andgood electron generation properties, may be used for the emissivecoating 174 in some embodiments of the invention. In one embodiment ofthe invention, the emissive coating 174 is a CsI layer with a thicknessof 100 nm. In another embodiment of the invention, the emissive coating174 is a CsI layer with a thickness of 5 nm.

Returning to FIG. 3 , in some embodiments of the invention, there willbe a voltage applied to the photoemissive structure using electricalcontact 176 and electrical lead 142. The relative voltage compared tothe voltage applied to the subsequent electron optics, such as cathodelens 152, will be set such that the emitted electrons 179 areaccelerated away from the surface of the photoemissive structure 170towards the electron optics.

It will be known to those skilled in the art that other architecturesfor the photoemissive structure can be designed comprising additionallayers, and in which voltage differences between the layers of thephotoemissive structure 170 are established as well, to accelerate theelectrons between the layers of the photoemissive structure 170.

Some embodiments of the invention will have a voltage controller 140that uses electrical lead 142, connecting to the photoemissive structure170, and cathode lens electrical lead 144, connecting to the cathodelens 152, to set the relative voltage of the photoemissive structure 170and the cathode lens 152. If the voltage provided through the lead 142to the electrical contact 176 is significantly more negative than thevoltage provided through the lead 144 to the cathode lens, then theemitted electrons 179 will be accelerated away from the photoemissivestructure 170 and into the electron optical system. In some embodimentsof the invention, a voltage difference of twenty kilovolts (20 kV) willbe established between electrical lead 142 and cathode lens electricallead 144. In another embodiment of the invention, a voltage differenceof fifty kilovolts (50 kV) will be established.

A typical electron optical system comprises a combination of electronoptics, such as the cathode lens 152, apertures 154, beam steeringoptical elements 156 and transfer and projection lenses 158. Theelectron optics can be positioned inside the vacuum chamber 120, such aswhen the electron optical design uses electrostatic lenses, or bepositioned outside the vacuum chamber 120, such as when magnetic lensesare used, as illustrated in FIG. 3 . The optics can also be configuredto be adjustable for imaging properties such as astigmatism and otheraberration corrections, and in particular to adjust the position andorientation of the cathode lens 152 relative to the photoemissivestructure 170. These can be predetermined adjustments, or may beadjusted by signals from the system controller 110 in response tofeedback about the imaging performance of the system. In someembodiments of the invention, the stage controls 132 may be used on thestage 130 holding the photoemissive structure 170 to adjust the positionand orientation of the photoemissive structure 170 relative to theelectron optics and the optical axis of the electron optics.

In some embodiments of the invention, the electron optics will comprisebeam steering optical elements 156 so that the emitted electrons 179 areno longer co-linear with the unabsorbed beam of x-rays 511. This allowsthe unabsorbed beam of x-rays 511 to fall into a beam dump 108, where itis absorbed and therefore prevented from leaving the vacuum chamber,reducing the risk of inadvertent radiation exposure.

In some embodiments of the invention, the various electron opticalelements form a magnified electron image 159 of the emitted electrons179 in the final image plane of the electron optical system. In someembodiments, the magnified electron image 159 will be 150 times largerthan the pattern of emitted electrons 179. In other embodiments, themagnified electron image 159 will be 1,500 times larger than the patternof emitted electrons 179.

The electron optical system will typically be designed such that theimage plane is within the vacuum chamber 120. An image converter 180 isplaced at this image plane that emits photons 198 when excited byenergetic electrons of the magnified electron image 159. These photons198 are generally visible photons (i.e. with a wavelength between400-700 nm), although in some embodiments the emitted photons 198 may beinfrared or ultraviolet photons. Some of the emitted photons 198 fromthe image converter 180 exit the vacuum chamber 120 through exit window127, and are collected by the imaging system 190. In some embodiments,this imaging system 190 comprises a video system or a CCD array tocreate electronic signals corresponding to the emitted photons 198.

FIG. 6 illustrates a detailed cross-section view of the structure of theimage converter 180 and imaging system 190 for this embodiment of theinvention. In this embodiment, the image converter 180 comprises ascintillator 184. Such scintillators are common in electron microscopy,and many variations containing a variety of phosphors that emit photonswhen stimulated with energetic electrons will be known to those skilledin the art. The scintillator 184 can comprise a phosphorescent material,such as zinc sulfide (ZnS) doped with manganese (Mn) or other elements,a structure comprising a crystal material such as Yttrium AluminumGarnet (YAG), or compositions comprising various rare earth elements.The fabrication of the image converter 180 should result in a uniformstructure of material in the scintillator 184, so that the intensity ofthe emitted photons 198 is in proportion to the number of incidentelectrons absorbed from the magnified electron image 159.

In some embodiments, the image converter 180 may comprise additionallayers, such as a conducting layer 186 on the side of the imageconverter 180 on which the electrons of the magnified electron image 159are incident. In some embodiments of the invention, this conductinglayer 186 can be attached electrically using an electrical lead 182 toset the image converter 180 to a specific voltage. In some embodimentsof the invention, the specific voltage on the electrical lead 182 willbe set to zero volts, and the lead 182 therefore provides a path toground for the absorbed electrons of the magnified electron image 159.In some embodiments, the voltage may be set by voltage controller 140.In some embodiments, the conducting layer 186 is fabricated using amaterial that reflects photons such as a metallic thin film. In someembodiments, the conducting layer 186 will be approximately 50 nm thick,and fabricated using a material comprising aluminum. This provides anadditional benefit of taking any photons from the scintilator 184emitted in the direction of the incoming electrons and reflecting themback towards the exit window 127, where they add to the intensity of theemitted photons 198.

Outside the vacuum chamber, an imaging system 190 can be used to producean image 199 of the emitted photons 198. In some embodiments, thisimaging system 190 comprises a lens system 192, an image sensor 194, andimage processing electronics 196 that can be used to convert the image199 of the emitted photons 198 into electronic signals. In someembodiments, the lens system 192 forms a magnified image of the emittedphotons 198. In some embodiments, this magnification is by a factor of100. In some embodiments, the image sensor 194 will be a charge-coupleddevice (CCD) array. In some embodiments, the signals will be a representthe image using video formats. In other embodiments, these signals willbe a collection of still images.

If the materials of the photoemissive structure 170 and the imageconverter 180 are well selected and uniformly fabricated, and theadjustments of the electron optics 152, 156 and 158 are made to minimizeaberrations and distortions, the final electronic signals from theimaging system 190 will represent a magnified image of the x-raytransmission of the corresponding portion of the integrated device 160.

FIG. 7 illustrates in more detail a schematic of the control systems tobe used for some embodiments of the invention. The electronic imagesgenerated by the imaging system 190 can be transmitted to the systemcontroller 110. This controller 110 can comprise a means for governingthe source of x-rays 100, such as adjusting the x-ray intensity, pulsingthe source, or making adjustments to the beam shaping optics 104 tocollimate or concentrate the beam.

This controller 110 can further comprise a means for electronic inputand output 114.

This controller 110 can further comprise an electronic processor 116.This processor 116 may be programmed to manage the external stagecontroller 234 that drives the external stage controls 232 that adjustthe position and orientation for the external stage 230 supporting theintegrated device 160. This processor 116 may also be programmed tomanage the stage controller 134 that drives the stage controls 132 thatadjust the position and orientation for the stage 130 supporting thephotoemissive structure 170. This processor 116 may be programmed tomanage the voltage controller 140 that adjusts the relative voltage ofthe photoemissive structure 170 and the cathode lens 152, and may alsocontrol the voltage setting for the electrical lead 182 for thescintillator 184. This processor 116 may also be programmed to adjustthe settings and aberration controls of the cathode lens 152.

In some embodiments of the invention, the controller 110 will alsocomprise electronic data storage 118, which can be used to record theposition and orientation set for the external stage 230, the stage 130,and the control voltages for the photoemissive structure 170 and imageconverter 180, as well as the corresponding images collected by theimaging system 190.

In some embodiments, the information and signals representing imagesrecorded in the electronic data storage 118 can be combined tosynthesize a two-dimensional (2-D) or three-dimensional (3-D)representation of the integrated device 160 or portions thereof.

In some embodiments of the invention, these synthesized 2-D or 3-Drepresentations can be compared with a stored representation of anintegrated device known to be correctly manufactured, or a databaserepresentation of the design rules or the layout of the device asdesigned, and the resulting comparison used to evaluate the attributesof the integrated device 160 being examined. Such a system can be usedas an inspection system for manufacturing quality control.

Second Embodiment of the Invention

FIG. 8 and FIG. 9 illustrate another embodiment of an apparatusaccording to the invention. FIG. 8 illustrates a cross section view ofthe overall system, and FIG. 9 illustrates a cross-section in detail ofthe integrated device 160 and photoemissive structure 170.

As in the previously described embodiment of the invention, an x-raysource 100 produces a beam of x-rays 111 which are partially absorbed bythe integrated device 160 under examination, forming a modified beams ofx-rays 211.

As in the previously described embodiment of the invention, theintegrated device 160 is mounted outside the vacuum chamber 120, usingan external stage 230. In some embodiments of the invention, thisexternal stage 230 will be fixed in place, and in others the externalstage 230 may have external stage controls 232 to adjust position andorientation. Adjustment of the external stage controls 232 can begoverned by an external stage controller 234, which is generally asystem of electronics also outside the vacuum chamber 120. In someembodiments, this stage controller 234 can in turn be controlled by theoverall system controller 110, which can designate an organized scan ofpositions and angles of the external stage 230 to facilitate theexamination of the integrated device 160.

After passing through the integrated device 160, the modified beam ofx-rays 211 enters the vacuum chamber 120. However, in this embodiment ofthe invention, the photoemissive structure 170 has been fabricateddirectly onto the support window 225 of the vacuum chamber 120. Thesupport window 225 may be similar in design and fabrication to thewindow 125 described in the previous embodiment, and will also be madeusing a material transparent to x-rays, such as beryllium or diamond,but may also need to be of a different thickness or composition to serveas both a window for the vacuum chamber 120 and also as a mechanicalsupport for the photoemissive structure 170.

This configuration has some advantages, in that the need for stage 130,stage controls 132 and stage controller 134 inside the vacuum chamberare eliminated, along with the corresponding feedthrough junctions 123.Also, the need to select two materials, the window 125 and the supportstructure 270, for mechanical and x-ray transmission properties issimplified to the selection of a single material for support window 225.

In some embodiments of the invention, a vacuum chamber may be designedin which the position and orientation of the window can also be adjustedrelative to the electron optics and the optical axis of the electronoptics. However, since windows for vacuum systems are typically fixed inplace, in the embodiment as illustrated here, the photoemissivestructure 170 also becomes fixed in position and orientation. Anyrelative changes in position or orientation angle between the integrateddevice 160 and the photoemissive structure 170 would then need to becontrolled through the position and orientation of the external stage230 for the integrated device 160.

Likewise, because the support window 225 will function as a seal for thevacuum chamber 120 and therefore be near or in contact with the walls ofthe vacuum chamber 120, care must be taken in setting the voltage forelectrical lead 142 relative to the electron optics so that electricalshorting through to vacuum chamber 120 does not occur.

As in the previously described embodiment of the invention, aftertransmission through the photoemissive structure 170, the unabsorbedbeam of x-rays 511 can proceed in the vacuum chamber 120 to a beam dump108 where it is absorbed.

As in the previously described embodiment of the invention, the emittedelectrons 179 are directed by a set of electron optics to form amagnified image 159 at an image converter 180. As in the previousembodiment of the invention, the photons 198 emitted by the imageconverter 180 leave the vacuum chamber through exit window 127 and areconverted to electronic signals in imaging system 190. These aretransmitted to a controller 110, which can record these images usingelectronic data storage 118.

As in the previously described embodiment of the invention, theinformation and signals representing images recorded in the electronicdata storage 118 can be combined to synthesize a two-dimensional (2-D)or three-dimensional (3-D) representation of the integrated device 160or portions thereof.

As in the previously described embodiment of the invention, thesesynthesized 2-D or 3-D representations can be compared with a storedrepresentation of an integrated device known to be correctlymanufactured, or a database representation of the design rules or thelayout of the device as designed, and the resulting comparison used toevaluate the attributes of the integrated device 160 being examined.Such a system can be used as an inspection system for manufacturingquality control.

Third Embodiment of the Invention

FIG. 10 through FIG. 13 illustrate another embodiment of an apparatusaccording to the invention. FIG. 10 illustrates a cross-section view ofthe overall system, and FIG. 11 illustrates a cross-section in detail ofthe integrated device 160 and photoemissive structure 170. FIG. 12 andFIG. 13 each illustrate a cross-section in detail of the integrateddevice 160 and photoemissive structure 170 for two different variationsof the embodiment.

As in the previously described embodiments of the invention, an x-raysource 100 produces a beam of x-rays 111. In this embodiment, however,the beam of x-rays directly enters the vacuum chamber 120 through theentrance window 125 without passing through the integrated device 160,becoming the interior unmodified beam of x-rays 411. The modificationsthat make the interior unmodified beam of x-rays 411 different from theincident beam of x-rays 111 are only due to absorption and scatteringfrom the window 125.

In this embodiment, the photoemissive structure 170 is depositeddirectly onto the integrated device 160 to be examined. Both theintegrated device 160 and the attached photoemissive structure 170 areentirely contained within the vacuum chamber 120. Therefore, the needfor external stage 230, external stage controls 232, and external stagecontroller 234 are eliminated. However, stage 330 within the vacuumchamber 120 is now used to hold both the integrated device 160 and thephotoemissive structure 170, and to adjust their positions andorientation angle relative to the interior beam of x-rays 411 as well.The design of the stage 330 and the stage controls 332 may be verysimilar to the stage 130 and stage controls 132 in the previousembodiments. However, the additional thickness and support requirementsfor holding both the integrated device and the photoemissive structuremay require some variation in design.

In some embodiments, both the position and the orientation of theintegrated device 160 may be adjustable using stage controls 332 for thestage 330, making it possible for a relatively small beam of x-rays tobe used to examine the entire area of an integrated device 160 muchlarger than the diameter of the slightly modified x-ray beam 411.Adjustment of the stage controls 332 can be governed by a stagecontroller 334, which is generally a system of electronics outside thevacuum chamber 120. In some embodiments, this stage controller 334 canin turn be controlled by an overall system controller 110, which candesignate an organized scan of positions and angles of the stage 130 tofacilitate the examination of the integrated device 160. In someembodiments of the invention, the stage controls 332 may be used on thestage 330 holding the integrated device 160 and photoemissive structure170 to adjust the position and orientation of the photoemissivestructure 170 relative to the electron optics and the optical axis ofthe electron optics.

As in the previously described embodiments of the invention, thephotoemissive structure 170 may comprise a layer 172 of photoemissivematerial and an emissive coating 174. However, in this embodiment theneed for an additional support structure 270 for the photoemissivestructure 270 is eliminated, since the structure 170 has been depositeddirectly on the integrated device 160.

FIG. 12 illustrates a variation of this embodiment of the invention. Inthe previously illustrated embodiments, the angle of incidence of thebeam of x-rays 111 and therefore also modified x-ray beam 411 isperpendicular (i.e. has an angle of incidence at or near 90°) to thesurface of the integrated device 160. In this variation of theembodiment, motion of the x-ray source 100 or adjustment of thebeam-shaping optics 104 can provide x-rays incident on the integrateddevice 160 at some value θ which is not 90°, and in fact in someembodiments may be adjustable to provide a multiplicity of angles ofillumination, either simultaneously or in a programmed time sequence.The angled beam of x-rays 611 will have an intensity pattern that isdifferent from the normal incidence case of FIG. 11 , and therefore thetrajectory and intensity pattern of the cascade of electrons 777 and theamplified electron cascade 778 will be different from the trajectory andintensity pattern of the cascade of electrons 177 and the amplifiedelectron cascade 178 in the normal incidence case of FIG. 11 .

However, once the electrons 179 are emitted from the surface of thephotoemissive structure 170, they are accelerated towards the cathodelens 152, and a magnified image is formed by the image converter 180 andimaging system 190, as in the previous embodiments of the invention.

Such an embodiment can be used in conjunction with various imageprocessing algorithms such as those for computed laminography, alsoknown as digital tomosynthesis, synthetic laminography, or computerizedsynthetic cross sectional imaging, in which images from multiple anglesare collected to and processed to provide a 3-dimensional representationof the layers of the integrated device 160. In some cases, a simpleparallax computation from two images at different angles may be enoughto infer 3-D structural information. In other cases in which the basicstructure (i.e. layer thicknesses and approximate feature sizes) areknown, collecting images for a few multiple angles near perpendicularmay provide enough information to infer 3-D detailed structuralinformation.

The advantage of computed laminography algorithms over more commonlyused computed tomography (CT) algorithms is that transmissioninformation from a wide range of angles around the sample need not becollected. When the integrated device 160 is in a vacuum chamberrequiring a window 125 for x-ray transmission, and the alignment withthe electron optics can be delicate, the wide range of motion requiredby many tomography algorithms can require a system that is mechanicallycomplex. When only a few angles and views are required, the integrateddevice 160 and photoemissive structure 170 can remain aligned with theelectron optics, and even at times immobile, and only the angle ofincidence of the beam of x-rays 611 need be changed.

Although we describe the integrated device 160 and photoemissivestructure 170 as being in “direct contact” in this embodiment, it willbe known to those skilled in the art that there may be someconfigurations in which it may be best to deposit additional layers ofbuffer material between the integrated device 160 and the photoemissivestructure 170, to provide a more physically flat, chemically neutral, orelectrically insulating surface. Such a planarization layer 370 isillustrated in FIG. 13 . This may be especially important if particularvoltage settings are desired for the electrical lead 142, since the leadmay be also in contact or close proximity to the integrated device 160and the stage 130.

As in the previous embodiments, some x-rays from the unmodified beam ofx-rays 411 are absorbed or scattered in the integrated device, and thenstimulate the emission of electrons 179 from the photoemissive structure170.

As in the previously described embodiments of the invention, aftertransmission through the photoemissive structure 170, the unabsorbedinterior beam of x-rays 511 can proceed in the vacuum chamber 120 to abeam dump 108 where it is absorbed.

As in the previously described embodiments of the invention, the emittedelectrons 179 are directed by a set of electron optics and form amagnified image 159 at an image converter 180. As in the previousembodiment of the invention, the photons 198 emitted by the imageconverter 180 leave the vacuum chamber through the exit window 127 andare converted to electronic signals in imaging system 190. These aretransmitted to a controller 110, which can record these images usingelectronic data storage 118.

As in the previously described embodiments of the invention, theinformation and signals representing images stored in the electronicdata storage 118 can be combined to synthesize a two-dimensional (2-D)or three-dimensional (3-D) representation of the integrated device 160or portions thereof.

As in the previously described embodiments of the invention, thesesynthesized 2-D or 3-D representations can be compared with a storedrepresentation of an integrated device known to be correctlymanufactured, or a database representation of the design rules or thelayout of the device as designed, and the resulting comparison used toevaluate the attributes of the integrated device 160 being examined.Such a system can be used as an inspection system for manufacturingquality control.

Other Embodiments of the Invention

FIG. 14 illustrates a cross-section in detail of the integrated device160 and photoemissive structure 170 for another embodiment of anapparatus according to the invention.

In this embodiment, both the integrated device 160 and the photoemissivestructure 170 are within the vacuum chamber 120, but the integrateddevice has a connected stage 430 with connected stage controls 432 thatare attached to the stage 130 and stage controls 132 for thephotoemissive structure 170. The connected stage 430 may be designed toallow the easy and rapid insertion of integrated devices 160, and toadjust their position and orientation not only with respect to theinterior unmodified beam of x-rays 411, but also relative to thephotoemissive structure 170. In some embodiments, the connected stage430 will be designed to allow the integrated device 160 to be moved tobe in very close proximity to the photoemissive structure 170. In someembodiments, photoemissive structure also has an independent support470, which may be similar in design and material composition to thesupport structure 270 described in the previous embodiments.

The advantage to such a configuration is that the device 160 and thestructure 170 can be placed in relatively close proximity, minimizingthe distortion from propagation and scattering that can occur withpropagation, without actually being in mechanical and electricalcontact. In some embodiments, the position and orientation angle of theintegrated device 160 can be adjusted independently.

The disadvantage to such a configuration is that both the device 160 andthe structure 170 must now have either independent mounting systemswithin the vacuum chamber 120, or a well designed single stage formounting that will allow the integrated device 160 to be inserted inclose proximity to the photoemissive structure 170 and its independentsupport 470, and also allow its removal, without damaging or misaligningthe photoemissive structure 170. The design of such mounting systems canbe costly, especially when required to be used entirely within a vacuumchamber.

There are other design concerns for the joined stage 130, stage control132, and connected stage 430 and its stage controls 432. If thephotoemissive structure is to remain in a stable position relative tothe electron optics, which is better for distortion control, then thedesign of the connected stage 430 holding the integrated device must beoffset with enough distance from the photoemissive structure 170 so thatchanges in the relative orientation angle and position of the integrateddevice 160 relative to the beam of x-rays 411 could be made without thedevice coming in contact with the structure 170, disrupting thealignment with the electron optics.

In an alternative embodiment, the integrated device 160 could be mountedin close proximity to the photoemissive structure 170 and itsindependent support 470 and the motions of the stage 130 and theconnected stage 430 rotated together if various orientation angles forthe integrated device relative to the interior unmodified beam of x-rays411 is desired. This may reduce the potential distortions caused bygreater distance between device 160 and structure 170, but may increasethe distortions caused by potential misalignments between thephotoemissive structure 170 and the electron optics. Also, unless thephotoemissive structure 170 were the same dimensions as the integrateddevice, a translation of the device 160 in x-y coordinates relative tothe structure 170 may be required, so that the entire device caneventually be observed. To increase signal strength, a high x-ray fluxis desired, and spreading the x-ray beam to cover the entire integrateddevice will reduce flux considerably.

Performance: Speed and Resolution

Given the descriptions above, the time to collect an image from a 1 cm×1cm integrated device can be estimated, and compared to prior art Fresnelzone plate (FZP) systems such as those previously described.

The relative imaging throughput of this system can be estimated usingthree factors:

-   -   1. Flux of x-ray illumination    -   2. Contrast in the specimen under examination    -   3. Detection efficiency.

As noted above, the spectrum of the x-ray source used with the disclosedinvention can be a broadband source, and in particular one in which asignificant fraction of the x-rays have higher energy than the copperK-absorption edge. The source brightness can be as high as 5×10¹⁰ x-rayphotons/mm² srad, while the brightness in the FZP system is at least afactor of 10 smaller, since only the characteristic 8 keV copper Kαfluorescence photons are used.

Also, the numerical aperture (NA) of the system disclosed here can beapproximately 9 mrad, while the NA of a FZP system is typically 3 mrad.The reduction in angle by a factor of 3 leads to a reduction in theamount of x-ray photons that can be collected and used to illuminate thespecimen, reducing incident flux by a factor of 9.

These two differences alone lead to an increase in throughput by afactor of at least 90 due to the increased incident x-ray flux for theinvention disclosed here.

The second factor affecting imaging speed (throughput) is imagecontrast. The imaging contrast (signal) depends on sample materials andx-ray energy (wavelength). As mentioned above, a FZP typically systemuses 8 keV copper Kα fluorescence as its x-ray source, to which copperinterconnects in an integrated device are mostly transparent (as wasillustrated in FIG. 5 ). By using broadband light, with a significantportion with energy greater than the copper K-edge absorption at 8.9keV, the contrast can increase by a factor of 15 or more. For an IC witha 50 micron thick silicon substrate, this corresponds to a 7× increasein signal contrast for a 1 micron thick copper line. The correspondingincrease in throughput is a factor of 7² or 49×.

The overall detector quantum efficiency (DQE) for the for the systemaccording to the invention (factoring in the conversion from x-rays toan electron cascade in the photoemissive structure 170, the conversionof electrons to photons by the image converter 180, and then toelectronic signals in the imaging system 190) is similar to that ofexisting FZP systems, about 2%. Therefore, the overall improvement inimaging speed is found by multiplying the increase in throughputs due toincreased x-ray flux (90×) and image contrast (49×). This leads to anoverall improvement in throughput for the disclosed system over theprior art FZP of 90×49=4,410.

Prior art FZP systems have been designed to inspect details of anintegrated device, but not for high-resolution examination of entireICs. To form a complete image of a 1 cm×1 cm IC using such a prior artsystem would take 200,000 hours (22.8 years). However, using theimprovements of a system according to the invention disclosed here, datacollection could occur 4,410 times faster, and a complete image could becollected in 45.3 hours—less than 2 days.

Of course, speed is not the only metric for such a microscope orinspection system. For integrated devices with 20 nm features, aresolution of 20 nm is desired. The resolution of a system according tothe invention is partly determined by the diffusion of the cascade ofelectrons 177 in the photoemissive structure 170. This diffusion reducesthe localization of the electron excitation, and causes a blur in theimage.

This diffusion depends on the geometry and material composition of thephotoemissive structure 170. A thicker structure will increasediffusion, creating more blur. Previous studies of x-ray-electron hybridimaging systems [L. A. Bakaleynikov, E. Yu. Flegontova, and E.Zolotoyabko, “Combined X-ray-electron Imaging Techniques: Limitations onLateral Resolution,” Journal of Electron Spectroscopy and RelatedPhenomena, Vol. 151, pp. 97-104 (2005)] indicate that excitation of aphotoemissive thin film of gold by an x-ray point-source can produceelectron emission with a resolution as small as 20 nm. The electronoptics can be designed such that they faithfully maintain thisresolution without further degradation.

Further Extensions and Limitations

Although this disclosure presents an apparatus for the microscopicexamination of integrated devices, and in particular copper integratedcircuit structures, it will be recognized that the term “integrateddevices” as used here can represent any manufactured object with small(e.g. micro- or nano-scale) features, such as silicon interposers withthru-silicon-vias (TSVs), packages containing multiple integratedcircuits (3D-IC structures), especially those with TSVs to connect themvertically, MEMS and NEMS devices such as micro-actuators andmicro-sensors, RF antenna structures, integrated optical devices,multi-function IC packages for cellular phones, photomasks,metamaterials, magnetic storage devices, and others that will be knownto those skilled in the art.

It will also be recognized that the apparatus disclosed here can be usedfor the examination of objects other than manufactured integratedstructures. Such objects can include mineral formations or biologicaltissue samples, especially biological tissue samples that may have beenmetallized for enhanced contrast. As long as the wavelength range forthe beam of x-rays is selected such that there is measurable contrast inthe absorption or scattering of x-rays from the internal structures ofthe object under investigation, a system as disclosed here can be usedto investigate these internal structures as well.

It will also be recognized that this microscope can be used as acomponent of an inspection system, in which the representations of the2-D and 3-D structures are compared with stored reference data. Thesedata can be either a reference image or set of images from a similardevice known to have been properly manufactured (a “Golden Image”), ordata from a similar region in the integrated device previously that hasbeen measured, or from a database representing the integrated devicedesign rules or geometric structures as designed.

With this application, several embodiments of the invention, includingthe best modes for various circumstances, have been disclosed. It willbe recognized that, while specific embodiments may be presented,elements discussed in detail only for some embodiments may also beapplied to others. For example, the image collection discussed in detailonly for the first embodiment can be applied to other embodiments aswell. Likewise, the angular variation of the x-rays described in detailin the third embodiment may find application in other configurations.

While specific materials, designs, configurations and fabrication stepshave been set forth to describe this invention and the preferredembodiments, such descriptions are not intended to be limiting.Modifications and changes may be apparent to those skilled in the art,and it is intended that this invention be limited only by the scope ofthe appended claims.

I claim:
 1. A system for computed laminography, comprising: a source ofx-rays; a vacuum chamber; a window that allows the transmission ofx-rays into the vacuum chamber; a stage for holding at least oneintegrated device, positioned inside the vacuum chamber; a photoemissivestructure within the vacuum chamber designed to emit electrons whenexposed to x-rays; a means for directing the x-rays from the source ofx-rays through the window and onto at least one integrated device heldon the stage; a means of adjusting the position and orientation angle ofthe x-rays relative to the stage holding the integrated device; anelectron optical system within the vacuum chamber, designed to form amagnified image of electrons emitted from the photoemissive structure; ameans of converting the magnified image into one or more electronicsignals; a means of recording the electronic signals corresponding tothe images; a system controller that directs at least: the means ofadjusting the position and orientation angle of the x-rays, motion ofthe stage holding the integrated device, and the means of recording theelectronic signals corresponding to the images; wherein the systemcontroller directs the means of adjusting the position and angle of thex-rays to produce images at multiple angles of incidence, allowingrecording of multiple sets of electronic signals corresponding to saidimages at a series of angles and positions; and a processor to determineinformation about the height of one or more features in the at least oneintegrated device held on the stage, the information determined usingthe multiple sets of electronic signals.
 2. The system of claim 1,wherein the system controller directs the means of adjusting theposition and angle of the x-rays to produce two images at two angles ofincidence.
 3. The system of claim 1, additionally comprising a means forcomparing the determined height of the features to a stored referenceimage.
 4. The system of claim 1, wherein the photoemissive structurecomprises material containing gold.
 5. The system of claim 1, whereinthe photoemissive structure comprises a layer of gold.
 6. The system ofclaim 5, wherein the thickness of the layer of gold is between 5 nm and100 nm.
 7. The system of claim 1, wherein the photoemissive structurecomprises material containing cesium iodide (CsI).
 8. The system ofclaim 1, wherein the photoemissive structure comprises a layer of cesiumiodide (CsI).
 9. The system of claim 8, wherein the thickness of thelayer of CsI is between 3 nm and 200 nm.
 10. The system of claim 1,additionally comprising: a means of adjusting the position andorientation angle of the photoemissive structure.
 11. The system ofclaim 1, wherein the means of converting the magnified image into one ormore electronic signals comprises: an image converter that absorbsincident electrons and emits photons, and an image collector to produceimages of the emitted photons.
 12. The system of claim 11, additionallycomprising: a second window that allows the transmission of the emittedphotons out of the vacuum chamber; and wherein the image collector toproduce images of the emitted photons is outside the vacuum chamber. 13.The system of claim 11, wherein the means of converting the magnifiedimage into one or more electronic signals additionally comprises a meansof converting the images of the emitted photons into electronic signals.14. The system of claim 1, additionally comprising: a means of storingthe one or more electronic signals.
 15. The system of claim 1, whereinthe integrated device is an integrated circuit.
 16. The system of claim1, wherein the integrated device is a silicon interposer withthrough-silicon vias.
 17. The system of claim 1, wherein the stage thatholds the integrated device also holds the photoemissive structure. 18.The system of claim 17, wherein the stage that holds the integrateddevice and the photoemissive structure has been designed so that theintegrated device can be inserted and removed from the stage.
 19. Thesystem of claim 18, wherein means is provided to adjust the position andorientation angle of the integrated device relative to the photoemissivestructure.
 20. The system of claim 18, wherein the stage that holds theintegrated device and the photoemissive structure has been designed sothat the integrated device can be inserted and positioned in closeproximity to the photoemissive structure.